A construction machine such as a hydraulic excavator is generally provided with a travel base which has a crawler belt or the like to travel, a turning upperstructure which is disposed above the travel base and connected thereto through a turning frame so as to turn in a left/right direction, and a work device which is provided in front of the turning upperstructure, such as a front work device which has actuators so as to perform work such as excavation. In addition, the construction machine has a counterweight which is provided at the rear of the turning upperstructure, and an engine room which is disposed in front of the counterweight.
The engine room has an engine, a heat exchanger which exchanges heat with the engine, an intake port which is formed in the exterior of the engine room to take in the outside air therethrough, a fan which introduces the outside air from the intake port into the engine room to cool the heat exchanger, and a tail pipe which is provided on the exterior of the engine room to release exhaust gas discharged from the engine to the outside.
Here, harmful nitrogen oxide is contained in the exhaust gas discharged from the engine. It is therefore necessary to reduce the nitrogen oxide to thereby decompose the nitrogen oxide into water and nitrogen and reduce the concentration of the nitrogen oxide contained in the exhaust gas before the exhaust gas is discharged to the atmosphere. To that end, the construction machine has an exhaust gas purifying device in the engine room. The exhaust gas purifying device reduces and purifies nitrogen oxide contained in exhaust gas.
For example, this exhaust gas purifying device includes an aqueous urea tank for storing aqueous urea, a reduction catalyst provided in an exhaust duct, and an injection device disposed in the exhaust duct on an upstream side of the reduction catalyst to inject the aqueous urea supplied from the aqueous urea tank into the exhaust duct. The aqueous urea injected by the injection device is hydrolyzed by the heat of the exhaust gas so that nitrogen oxide contained in the exhaust gas can be decomposed to harmless water and nitrogen and purified by reduction reaction between ammonia produced by the hydrolysis and the nitrogen oxide in the reduction catalyst.
For the aforementioned aqueous urea tank storing the aqueous urea that purifies the exhaust gas, various layouts have been proposed in consideration of the behavior of the aqueous urea, the dimensions of the aqueous urea tank etc. and in accordance with the usage site, purpose or the like of the construction machine. For example, since the melting point of the aqueous urea is about −11° C., the aqueous urea tank may be disposed near a device such as the engine or the hydraulic pump generating heat in the engine room so as to prevent the aqueous urea stored in the aqueous urea tank from being frozen due to the temperature decrease of the outside air. Alternatively, the aqueous urea tank may be disposed within the counterweight in order to acquire a space in the engine room.
Specifically, as one of background-art techniques about the construction machine provided with the aqueous urea tank, there has been known a construction machine in which a heat exchanger, a fan disposed to face the heat exchanger so as to generate cooling wind for accelerating heat release from the heat exchanger, and an aqueous urea tank for storing a liquid reductant, that is, aqueous urea to be supplied to an NOx reduction catalyst are provided in a machine room, and the aqueous urea tank is disposed on an upstream side of the heat exchanger in the flow direction of the cooling wind, in order to prevent the aqueous urea in the aqueous urea tank from reaching a high temperature (for example, see Patent Literature 1). In this construction machine, the aqueous urea tank releases heat to the cooling wind which has not yet passed the heat exchanger, so that the temperature rise of the aqueous urea in the aqueous urea tank can be suppressed while the aqueous urea can be prevented from being frozen.